II.

Types of Supernovas

How a supernova explodes and what kind of object is left by the explosion depends on basic features of the star itself.

A star forms from a cloud of gas and dust that contracts in on itself from the force of gravity. If the object at the center of the collapsing cloud has at least 8 percent the mass of the Sun, gravity can compress the gas in the core of the object to a point where the mutual repulsion of atomic nuclei is overcome and nuclear fusion of hydrogen into helium begins. In the process a small amount of matter is turned into large amounts of energy as expressed in Einstein’s famous equation E = mc² (energy equals mass times the speed of light squared). The energy released at the star’s core heats and pushes back the outer layers of gas, and the star begins to shine.

A star’s existence is a constant battle between gravity pulling gases in toward the center and energy pushing back. Over a star’s life, lighter elements are fused together into heavier elements. The star’s final fate is tied to how long it can sustain nuclear fusion. As a basic rule, the more massive the star, the hotter and faster it will burn up its nuclear fuel, and the more cycles of burning heavier elements it can reach. An extremely massive star may last only for a few million years, compared to the 10-billion-year life span of our Sun. See also Life Cycles and Ages of Stars: Star (astronomy).

In very large stars known as supergiants, the fusion processes can continue to create and burn nuclei of heavier elements including carbon, oxygen, nitrogen, neon, and silicon. Fusion of different elements occurs at different layers in the stars, giving them an onion-like structure. If these stars are less than 10 times the mass of the Sun, they generally will shed enough of their outer layers to end up as white dwarf stars. Supergiant stars that have at least 8 to 10 times the mass of the Sun can explode as supernovas.

A.

Classification of Supernovas by Light Spectrum

The spectrum of the light from supernovas is an important source of information that can be used to help classify supernova explosions. Spectral lines indicate what chemical elements are present and what temperatures occur in the explosion. These spectral lines vary as the brightness of a supernova fades.

Astronomers use the term Type I for supernovas that do not show spectral lines of hydrogen and Type II to describe supernovas that do. It is thought that Type I supernovas explode after the stars have lost their outer layers of hydrogen gas. Type II supernovas explode at a stage when the stars still have hydrogen gas in their atmospheres.

Scientists recognize a number of subtypes for Type I supernovas, known as Type Ia, Type Ib, and Type Ic. Like Type Ia supernovas, Types Ib and Ic lack hydrogen lines in their spectra. However, they result from a different explosion mechanism from Type Ia—core collapse of a giant star rather than from the thermonuclear explosion of a white dwarf star. Different types of supernovas also have distinct light curves or patterns of brightness over time. Type Ia supernovas are typically brighter than other supernovas, while Type Ib and Type Ic are generally dimmer.

The expanding debris from Type I supernovas gets its energy for shining from the radioactive decay of isotopes created in the explosion. Type II supernovas are thought to shine as the envelope of material that surrounded the giant star before it exploded is heated by the shock wave created in the core collapse event.

B.

Classification of Supernovas by Explosion Mechanism

Another way to classify supernovas is by how they explode. Two basic mechanisms involving gravitation are thought to cause a star to collapse onto itself, generating a supernova explosion: (1) core collapse and (2) excess matter added to a white dwarf, causing a thermonuclear explosion.

B.1.

Core-Collapse Supernovas

Giant stars that contain more than about 10 times the Sun’s mass can form elements all the way up to iron in their cores. When the iron fuses to form still heavier elements, the process uses up energy instead of releasing it. When no more energy is released in the star's core, the outward pressure that countered the inward pull of gravity disappears. The core of the giant star collapses in seconds. Gravity causes the rest of the star to crash in toward the core, resulting in a catastrophic explosion. Core-collapse supernovas can leave behind superdense neutron stars or black holes.

All Type II supernovas result from core collapse. Type Ib and Type Ic supernovas are also thought to explode from core collapse, but only after much of their outer layers were expelled. Type Ib supernovas have shed their hydrogen and show spectral lines of helium. Type Ic supernovas have also shed the helium layer, revealing the carbon and oxygen layer below by their spectral lines.

The term hypernova has been proposed for an extremely massive core-collapse supernova—possibly more than 100 times the mass of the Sun. A hypernova is thought to form a black hole. Just before it explodes, a hypernova may release a huge burst of gamma rays in a jet from the rotating black hole at its center. These jets may explain the so-called long gamma-ray bursts detected by astronomers. According to some researchers, massive stars with over 40 solar masses may sometimes collapse directly into a black hole without generating an explosion. Such an event might generate gravitational waves.

Because giant stars have short lives that last only a few million years, the huge stars that explode as core-collapse supernovas are only found in regions of galaxies that have populations of new stars. New stars can form in irregular galaxies and in the spiral arms of spiral galaxies, both places where core-collapse supernovas have been observed. However, core-collapse supernovas are not seen in elliptical galaxies, which are populated by older stars and lack active star-forming regions. The supernovas that occur in elliptical galaxies result from a different explosion mechanism, which can also cause supernovas in irregular and spiral galaxies: thermonuclear explosions.

B.2.

Thermonuclear Explosion Supernovas

A very different kind of supernova is thought to come from a white dwarf star that explodes when too much extra matter is added to its surface. The extra matter is drawn off a nearby companion star in a double-star system. The white dwarf cannot support the added mass and begins to collapse on itself. The mass limit at which this collapse occurs is 1.4 times the mass of our Sun and is called the Chandrasekhar limit after the scientist who proposed it.

The inward pull of gravity violently fuses the atomic nuclei in the white dwarf together, resulting in a thermonuclear explosion that incinerates the entire white dwarf. Only a nebula of gas and dust remains, called a supernova remnant. The heat from the radioactive decay of the unstable isotope nickel-56 created in the fusion processes makes the debris shine for weeks or months.

Such explosions appear as Type Ia supernovas and can occur in older star regions of all types of galaxies, including elliptical galaxies. Because Type Ia supernovas occur within very strict mass limits, all Type Ia supernovas explode with about the same level in brightness, making them very useful tools for astronomers. Because they all have the same luminosity, the apparent brightness of a Type Ia supernova indicates its distance.